1. Field of the Invention
The present invention relates to a belt transporting device for circulatingly transporting an endless belt, which is in use with an image forming apparatus, such as copying machine or a printer. More particularly, the invention relates to a belt transporting device which is effective in preventing an inclination of the belt and an image forming apparatus using the same.
2. Background Art
Recently, in the image forming apparatus based on the electrophotography, for example, demands for size reduction, picture quality improvement, and cost reduction are increasing. To satisfy the demands, it is effective to employ the belt unit in the intermediate transfer body, sheet conveying body, fixing unit and the like.
In the intermediate transfer type image forming apparatus based on the electrophotography system, for example, which is already proposed, toner images of respective colors are successively formed on a photo receptor, those color toner images are primarily transferred onto the intermediate transfer body in a superimposed fashion, and those superimposed color images on the intermediate transfer body are simultaneously transferred onto a recording medium.
In this type of image forming apparatus, as known, the photo receptor takes a drum unit, and the intermediate transfer body takes the form of a belt unit (belt transporting device). The term “belt transporting device” means such a device that an endless belt is laid on a plurality of tension rolls, and the belt is circulatingly transported in a given direction.
In this type of belt transporting device, the belt does not linearly run, but runs while being biased to the axial direction of the roll, and hence there is the possibility that the belt inclines to its displaced direction, viz., a called inclined running of the belt occurs. Various factors causing this phenomenon are present: dimensional tolerances of structural components forming the belt transporting device, for example, parallelism of the rotary shafts of a plurality of tension rolls for supporting the belt in a stretching fashion, roll outside diameter variation, and tension unevenness of the belt owing to a variation of the periphery length of the belt.
A conventional belt-biasing preventing technique is present. In the technique, ribs are provided over the entire length of at least one end of the inner surface of the belt. The ribs are brought into engagement (contact) with (or fit to) grooves or the ends of the tension rolls to thereby regulate the belt inclination (see Japanese Patent Laid-Open No. 57-76579, for example).
Another technique for the belt-biasing prevention is that a flange of which the diameter is larger than the outside diameter of the tension roll is provided at least one end of the tension roll, and the belt, when runs, is restricted in motion at the end by the flange to forcibly be corrected in its running direction (Japanese Patent Laid-Open No. 06-27835).
The conventional techniques stated above have the following technical problems.
In the former or first conventional technique (based on the ribs), if the belt is biased in its running direction to one side of the belt and the ribs engage with the engaging parts (grooves, roll end or the like), and in this state, the biasing force continuously acts on the belt for a long time, stress repeatedly concentrates on the root of the rib (boundary part of the inner surface of the belt at which the rib is attached). The root of the rib will be cracked, and in an extreme case, the rib root is peeled off and the belt is seriously damaged.
The belt runs in a state that the ribs are constantly pressed against with the belt. In this state, non-uniformity of rib bonding accuracy will cause undulation and tilting in the running belt, so that the running belt will meander. When the belt meanders, the color toner images which are successively transferred onto the belt or the recording medium supported on the belt are shifted from the correct positions. As a result, a color picture finally formed on the recording medium suffers from image defects, such as color misregistration and hue variation.
The work of joining (bonding) of the ribs is troublesome, from the very beginning. Apart from this, to avoid the meandering of the running belt, it is essential to join (bond) the ribs to the belt with high precision. The rib bonding leads to cost increase, however, and in this respect, it is not a desirable measure.
In the second conventional technique (based on the flange), the end of the running belt is restricted by the flange to run following the belt end. When the running belt is biased and the biasing force continuously acts on the running belt in a state that the belt end is in contact with the flange, stress acts on the belt end and as a result, the belt is deformed to float up by the flange, viz., an undulation occurs in the belt. The undulation will crack the belt end, and in an extreme case, the belt is broken.
Even in a case where no undulation occurs, the flange frictionally slides on the side face of the belt end continuously, wear grows and hence, the durability performance is deteriorated.
An additional belt-biasing preventing technique is also proposed in which the roll is used in association with the belt surface in addition to the flange and ribs (Japanese Patent Laid-Open Nos. 10-282751 and 11-161055).
This technique is still unsatisfactory in solving the belt biasing problem since is complicated in construction and high in cost. While a chance of the belt cracking and damaging owing to the contact of the belt with the flange or ribs is lessened, indeed, the possibility that the belt is undulated by the end of the roll, and cracked and damaged is still present.
This possibility is great in particular where the biasing force is great. Further, certain accuracy is required for dealing with such a factor as parallelism.
A further belt-biasing preventing technique is proposed in which a tapered roll is disposed on the inner surface of the belt, and corrects the running belt biased outside from the end part of the tension roll (Japanese Patent Laid-Open No. 11-79457).
This technique also requires certain accuracy for the placement of the tapered roll, and is complicated in construction. A possibility that the undulation, cracking and damaging of the belt occurs in a gap part between the tension roll and the tapered roll is present. Accordingly, certain degree of accuracy is required for the belt-biasing causing factor, such as a parallelism of the roll, as in the previous techniques.
As described above, the conventional belt-biasing preventing techniques still have technical problems to be solved: at the contact part of the belt where it contacts with the regulating member, such as the flange or the ribs, the end part of the auxiliary roll, and the gap, the stress generated therein by the biasing force give rise to the undulation, meandering, cracking, damaging of the belt.
In the case additionally using the auxiliary roll, the construction is further complicated, and disadvantageous also in the light of cost.